The present invention generally relates to a high frequency heating arrangement and more particularly, to a heat generating vessel or container for use in a microwave oven which is one example of high frequency heating apparatuses, so as to generate heat through projection of microwaves irradiated from a magnetron (i.e., a high frequency generating means) for heating an object to be cooked, and also for baking thereof.
A microwave oven is a cooking apparatus arranged to guide microwaves emitted from a magnetron into an oven interior or heating chamber for irradiation onto an object to be cooked so as to effect cooking by causing said object itself to generate heat.
However, there are some items to be cooked which are not suitable for direct heating by microwaves such as those requiring scorched portions or those to be subjected to cooking after expediting fermentation by raising the temperature thereof.
In order to deal with the cooking items as referred to above, there has been proposed a microwave oven further provided with a sheathed heater in the heating chamber so as to make it possible to subject the item to be cooked to heat treatment through utilization of heat irradiated from said sheathed heater besides the microwave radiation.
In the microwave oven of the above described type, however, since two kinds of heating means, i.e., the magnetron and the sheathed heater, must be provided as heat sources, not only cost increase is involved thereby, but the construction of the microwave oven is undesirably complicated, with a consequent increase in the size of the apparatus on the whole.
Therefore, in order to overcome the various problems as described above, there has been recently developed a heat generating member composed of a plate in a double layer construction formed by laminating a heat generating substance (e.g., silicon carbide, ferrite or the like) which generates heat through irradiation of microwaves and an inorganic heat insulating base material (e.g., glass, ceramic or the like). There has also been proposed a heat generating member made of a silicon carbide group ceramic molded plate.
The microwave oven employing the heat generating members of the above described type is capable of effecting both the dielectric heating and the heating by heat radiation, only through irradiation of microwaves and is referred to as a multi-function microwave oven.
Incidentally, due to the fact that a so-called "home bakery" or household bread baking unit has recently become a popular article, a microwave oven provided with a bread baking function has been studied and manufactured as an actual product commercially available.
Although a bread baking container or hopper (referred to as a hopper hereinafter) to be disposed in the oven interior or heating chamber of the microwave oven is generally arranged to be heated indirectly, this practice requires parts for subjecting heated air to convection for efficient conduction of heat to the hopper, and thus, not only cost increase is involved thereby, but power consumption is undesirably raised due to poor heating efficiency, even when the heat is conducted in an efficient manner.
On the other hand, as a direct heating practice, there has conventionally been proposed an arrangement in which a microwave absorbing heat generating material is applied over an outer surface of a hopper as disclosed in Japanese Patent Laid-open Publication Tokkaisho No. 58-52916, or another arrangement in which a microwave absorbing heat generating material is coated on a ceramic or glass container as disclosed in Japanese Patent Laid-open Publication Tokkaisho No. 58-52917.
However, the known arrangements as described above have such problems that uneven baking (or scorching) may take place or yeast for fermentation is undesirably killed if applied to the bread bakery, since microwaves are transmitted into the container.
Meanwhile, since some of the conventional multi-function microwave ovens effect both functions for the microwave heating and the heating by heat radiation only by irradiation of microwaves, with the heating by heat radiation not killing yeast as in the microwave heating, they are applied to a bread baking device widely employed recently as well as to composite cooking. Such known bread baking device includes a hopper for accommodating materials such as flour and the like, to be disposed on a bottom plate in the heating chamber of the microwave oven, with the outer face of the hopper being coated by a microwave absorbing heat generating material, a kneading device having a stirring blade and provided under the hopper, and a metallic open-close lid of a clip fixing type provided at the upper portion of the hopper for shielding microwaves, and an outer cover made of a heat-resistance glass or the like for enclosing the above arrangement therein.
The known bread baking device as described above, however, has such disadvantages that, since the microwave absorbing heat generating material is coated over the entire outer surface of the hopper and the upper portion of the hopper is shielded by the metallic open/close lid, without provision of any turntable or stirrer fan, irradiation of the microwaves from the upper portion through the outer cover, onto the microwave absorbing heat generating material becomes uneven, and thus, there is a possibility that the heat generating material at portions subjected to much microwave irradiation is melted due to local overheating, while it is difficult to uniformly scorch the entire surface of the bread to be prepared. Moreover, owing to the fact that the heat generating material is directly coated onto the outer surface of the hopper, said material tends to be subjected to various damages, e.g., separation, cracking by sintering, etc., arising from handling, such as grasping of the hopper during taking out the bread, careless collision thereof with other utensils, washing of the hopper by brushes, etc. after use, thus presenting problems from the aspect of durability. Furthermore, since the open/close lid at the upper portion of the hopper is of the clip fixing type, there is such a possibility that the bread swelling at the finished stage is compressed by the open/close lid for bursting upon releasing of the clip.